Humidity sensors based on electric signals have become very popular in recent times. A humidity sensor measures the humidity content in the surrounding environment. More recently, polymeric films have been used as a humidity sensing element. Polymer-based humidity sensing elements can generally be classified into two categories: capacitance-type and impedance-type. The former typically involves more complicated circuit design and manufacturing process, and thus is more expensive, than the latter. A impedance-type electric humidity sensing element changes its electrical impedance as the humidity of the surrounding environment changes, and the measured impedance is converted into humidity readings.
The polymer-based sensing elements can also be further classified into two categories: porous (or more specifically, micro-porous) type, and non-porous type. There are numerous examples of porous polymer film based impedance-type humidity sensors. One of the examples is a microporous polyethylene film, in which 2-acrylamido-2-methylpropane sulfonic acid was graft-polymerized by ultraviolet irradiation. There are also numerous examples of non-porous polymer film based humidity sensors. One of the examples is a three-polymer-coated quartz crystal whose frequency varies as a function of the change in humidity. The first example is discussed in an article entitled: "Humidity Sensor Composed of a Microporous Film of Polyethylene-Grafted-Poly-(2-Acrylamido-2-Methylpropane Sulfonate)," by Y. Sakai, et al., Polymer Bulletin, 18, 501-506 (1987). The second example is discussed in another article entitled: "Relative Humidity Measurements Using A Coated Piezoelectric Quartz Crystal Sensor," Sensors and Actuators, 11 319-328 (1987). The three polymer coatings used in the second example include HEM-AMPS copolymer (HEM=2-hydroxyethyhnethyacrylate, AMPS=2-acryl-amido-2-methyl-propane sulphonic acid); cellulose acetate; and a modified epoxy resin. When moisture is absorbed by the polymer coating, the mass of the crystal is changed, causing its oscillating frequency to be changed.
The above two examples are specifically mentioned here because they both involve the use of PAMPS (poly(2-acryl-amido-2-methyl-propane sulphonic acid)) in the sensing element. In these examples, PAMPS itself is non-porous (as in Example 2). To make a porous polymer film, the PAMPS must be graft-polymerized into a micro-pourous polyethlene film. This unavoidably complicates the manufacturing process and increases the production cost.
Non-porous PAMPS can be used in making a impedance-type electric humidity sensing element by forming a PAMPS film on a pair of electrodes. However, the thickness of the non-porous PAMPS film must be carefully controlled within a very narrow range. If the PAMPS film is too thick, several undesirable effects, such as slow response time and significant hysteresis, will be experienced. On the other hand, if the PAMPS film is too thin, the impedance will be too high. Porous polymer-based sensing elements can provide the advantages of quick response time and small hysteresis. However, these advantages, at the present time, cannot be enjoyed by PAMPS-based sensing elements, without the extra step of grafting the PAMPS polymer into another polymer, such as polyethylene.